Journal of Geosciences, 56 (2011), 359–374

DOI: 10.3190/jgeosci.106

Original paper

Manganese-rich garnet–quartz rocks and gneisses in the Bohemianpart of the Moldanubian Zone: lithostratigraphic markers
Stanislav VRÁNA
Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic; stanislav.vrana@geology.cz
Manganese-rich garnet–quartz rocks and gneisses occur in the Varied Group of the Moldanubian Zone, Bohemian
Massif, in close association with amphibolites, marbles and accompanying graphite gneisses. Fine-grained garnets
contain (mol. %) 26–37 spessartine, 36.8–45.9 almandine, 11.1–14.3 pyrope and 2.9–21.0 grossular. Minor amphibole
present in some samples is ferrimagnesiohornblende with 0.17–0.22 Mn pfu. Accessory ilmenite contains 24–34 mol. %pyrophanite and 1.7–5.8 hematite. Some closely associated impure calcite marbles (or amphibolites) carry Ti-bearing
andradite, epidote, diopside–hedenbergite, and accessory magnetite.
Data from the Varied Group indicate that manganese enrichment took place both under oxidizing and reducing conditions, but the Mn-garnet–quartz rocks are oxidic. Normalization of major-element contents in the Mn-rich rocks
by average abundances in Varied Group paragneisses shows ten- to hundred-fold enrichment in MnO and a slightto moderate increase in CaO and P2O5. Values for Na2O and K2O indicate severe depletion in some samples, butcontents of other oxides are close to unity. Comparison of chondrite-normalized REE patterns in Mn-rich rocks withdata for ordinary paragneisses (Varied Group) also indicates that detrital component in Mn-rich rocks was closelycomparable to material supplied for protolith of paragneisses. This permits to ascribe the spike of Mn to likely shortlived exhalative processes.
Manganese-rich garnet–quartz rocks and gneisses remained unnoticed till now mainly owing to a small thickness of
their layers, typically less than 1 m. Structural relations in regions carrying Mn-rich rocks indicate their occurrence inrelict domains of preserved D1 structures with NW–SE trending foliation. It is suggested that in regionally prevalentareas dominated by superimposed refoliation (D2, D3) the thin layers of Mn-rich rocks were likely reduced to boudinsdispersed in paragneiss matrix. Comparison with published data for similar Mn-rich rocks abroad (including the so-called
coticules) strongly indicates a lithostratigraphic correlation potential of Mn-rich garnet–quartz rocks.
Keywords: Mn-rich garnet–quartz rocks, Ti-andradite–epidote marbles, exhalative processes, lithostratigraphic markers, Český Krumlov
Varied Group, Moldanubian Zone
Received: 27 June 2011; accepted: 1 December 2011; handling editor: E Jelínek

1. Introduction
Numerous volcanosedimentary units of various ages
worldwide carry sedimentary horizons rich in manganese,
grading in some cases to economic Mn deposits (Bühn
et al. 1992) or polymetallic deposits (Spry and Wonder1989). Garnet quartzites passing to garnet-rich rocks witha high content of spessartine are an occasional memberof lithological assemblage and were raised to the statusof exploration guide (Wonder et al. 1988). Following
the prevailing interpretation of garnetiferous rocks richin Mn as products of mixing of exhalative material
released from syn-volcanic vents at the sea-floor withdetrital material, the horizons of such rocks represent a
record of highly specific events during accumulation of
the volcanosedimentary column.
In the Czech part of the Moldanubian Zone in the
Bohemian Massif the occurrences of garnet–quartz rocks

and Mn-rich gneisses have received little attention (Vrána
1977, 1992) and remained largely unknown till present.
The main reason for this is the minimal thickness ofmetasedimentary Mn-rich horizon (horizons?), typicallyless than 1 m. This is unfortunate, as Mn-rich garnet–
quartz rocks potentially represent means of lithostratigraphic correlation in the complexly deformed gneisses
and migmatites of the Moldanubian Zone in southern
Bohemia, i.e., metamorphic complex so far resisting a
reliable lithostratigraphic classification. It is probably notby mere coincidence that most of occurrences reported
in this article are from domains with preserved NW–SE
to WNW–ESE structural trend, considered as the oldest
recognised structural pattern in the Czech part of the
Moldanubian Zone (Kodym 1972; Vrána 1979; Vrána andŠrámek 1999). Attention is also paid to associated rocks
documenting an increased oxygen fugacity environment,
i.e. rocks with an assemblage Ti-andradite + epidote +www.jgeosci.org

Stanislav Vrána
Mn-rich rocks
metamorphic rocks
Variscan granitoids

ral

nt
Ce

ia
hem

Bo

x

ple

om
cC
oni

lut

nP

Chýnov

Tábor
Hosty Doubravka

GermanynB
bianu

NovosedlyŠebanov

lda

20 km

Český
Krumlov

Austria

Mo

N

ath

oli

th

České Budějovice

Fig. 1 Localities of Mn-rich rocks in southern Bohemia, Czech
Republic.

magnetite, which occur in close proximity to the Mn-richgarnet–quartz rocks.

2. Geological setting

gneiss and amphibolite, is assumed to be mainly Neoproterozoic in age, as the protoliths to the orthogneisses
were dated between c. 550 Ma and 508 Ma (Vrána and
Kröner 1995; Košler et al. 1996). The Varied Group isprobably Cambro–Ordovician (Fiala et al. 1995; Drábek
and Stein 2003; Janoušek et al. 2008; Košler et al. 2011).
The Varied Group contains abundant layers of marble,
calc-silicate gneiss, graphite gneiss, quartzite and amphibolite in addition to dominating paragneisses.
There is some uncertainty concerning the internal
coherence of the Varied Group, as according to someinterpretations, it can be divided into the Český Krumlov
and Sušice–Votice groups, based on distinct lithology
and geographic distribution (Kodym and Suk 1958;
Jenček and Vajner 1968). In any case, an overview of thelocalities of manganiferous rocks is presented in Fig. 1
and simplified sketches of geological situation in theindividual areas in Fig. 2.

3. Localities
The list of studied localities with GPS co-ordinates isgiven in Tab. 1.

3.1. Pacova hora quarry near Chýnov

The Moldanubian Zone is a heterogeneous complex ofseveral major allochthonous units assembled during the
Variscan orogeny and modified by several superimposed
Variscan deformation and recrystallization events. The
principal members are the Monotonous Group, VariedGroup, and Gföhl Unit (Fiala et al. 1995). Allochthonous segments of older basement are also incorporated,
including the Palaeoproterozoic Světlík orthogneiss insouthern Bohemia (Wendt et al. 1993; Fiala et al. 1995)
and the Mesoproterozoic Dobra gneiss in Lower Austria(Gebauer and Friedl 1994; Friedl et al. 2004). The Monotonous Group, composed dominantly of biotite andsillimanite–biotite paragneisses, with widely scatteredorthogneiss bodies, subordinate quartzite, calc-silicate

The local sequence of the Varied Group (Fig. 2a) includes
calcite and dolomite–calcite marbles, rare metadolomites,
amphibolites, calc-silicate rocks, muscovite–biotite
gneisses, and minor quartzites. In 1985 quarrying operations exposed a 60 cm thick layer of amphibole–garnet
calc-silicate gneiss rich in Mn, directly bordering amphibolite layer (Fig. 3). Calcite marble within several metres of the Mn-rich layer is represented by a silicate-richvariety carrying aggregates of minerals rich in Fe3+ suchas andradite, epidote, accessory magnetite, in addition
to diopside (Figs 4–5). Relations of rock types in thequarry were characterized by Orlov (1931), geology ofthe broader area by Suk et al. (1977), mineralogy in thequarry and nearby caves by Litochleb et al. (1997, 2004),

Tab. 1 Sample locations of the studied Mn-rich rocks
Locality No.

Locality

Sample

WGS-84-N

WGS-84-E

Main minerals

1Chýnov

BD60a

49°25.84'

14°49.85'

Grt–Qtz with Pl and minor Hbl

1Chýnov

BD58

–"–

–"–

Ti-andradite–hedenbergite/diopside–calcite
epidosite/marble with Ti-andradite and diopside1Chýnov

CV1

–"–

–"–

2

Doubrava

TYV10849°16.36'

14°22.68'3Český Krumlov

13*

4

Novosedly

BD129

48°47.80'

14°15.20'

5Šebanov

VE127b48°45.30'

14°12.46'

Grt–Qtz

6Šebanov

VE127e–"––"–Ti-andradite–hedenbergite–Pl in amphibolite

13* – whole-rock chemical analysis reported by Bouška et al. (1985)

360

Grt–Qtz
Cal–Cpx–Pl
Grt–Phl–Prh–Gr–Qtz–PlManganese-rich garnet–quartz rocks and gneisses, Moldanubian Zone

Doubrava

1 km

ice R
iver

b

a

Lu n

Hoøice
Hosty
Kolodìje n. L.
Vl

Fig. c

ta

va

Ri

ve

Chýnov

Kladruby
0.5 kmr

Neznašov

Blanský
Blanský les
les granulite
granuliteNc
Lu nice River

d

Èeský
Krumlov

100 m

Novosedly
Hoøice na Šumavì

1 kmŠebanoveSvìtlík orthogneissparagneiss and migmatite

marblequartzite

calc-silicate gneiss

amphibolite

graphite gneiss

orthogneiss/granulite

locality of Mn-rich rocks

0.5 kmFig. 2 Simplified geology of areas with occurrence of Mn-rich rocks, based on 1:25 000 maps of the Czech Geological Survey. a – Chýnov;b – Hosty–Doubrava; c – pattern of graphite gneisses in the Hosty graphite deposit, based on documentation of exploration trenches (Tichý 1975);d – Český Krumlov–Novosedly; e – Šebanov. Note the prevalence of NW–SE trending structural patterns (D 1 generation).

while geochemistry and petrology of amphibolites wasstudied by Janoušek et al. (2008).
A tendency to local enrichment in Mn is independently
documented by rare occurrence of the violet variety oftremolite in marble (“hexagonite”, containing up to 0.29
wt. % MnO, Litochleb and Sejkora 2007).

3.2. Hosty–Doubrava villages near Týn nad
Vltavou
The occurrence of fine-grained garnet–quartz rock enriched in Mn (Fig. 2b) was briefly described by Vrána(1977). Associated are graphite–biotite quartzite and amphibolite. Some loose fragments occur 1 km NW of Hosty
361

Stanislav Vrána

Py

PQH

Grt
A

A

5 cm
village, not far of Hosty graphite deposit, Fig. 2b (Tichý
1975). West and SW of Koloděje nad Lužnicí, there isa relict domain of early, NW–SE trending D1 structures(Fig. 2b–c), overprinted in wider area by younger D 2
NNE–SSW structures (Fig. 2b). A detailed plan of graphite gneiss distribution, resulting from graphite exploration
campaign (Tichý 1975), and shown in Fig. 2c, serves as
an example of a domain with D1 structures still preserved.

3.3. Městský vrch graphite deposit in Český
Krumlov
Calc-silicate rock enriched in manganese was documented
by whole-rock chemical analysis reported by Bouška et al.

Fig. 3 Conformable contact of banded
garnet–quartz rock (with plagioclase andamphibole) with amphibolite. Grt – garnet
bands, PQH – plagioclase–quartz–hornblende bands, A – foliated amphibolitewith pyrite (Py) partly aligned parallelto the fold’s axial cleavage. Locality 1,
Pacova hora near Chýnov.

(1985). Location of this sample is not shown in Fig. 2d as
the accurate sampling site in Český Krumlov is uncertain.

3.4. Novosedly village near Český Krumlov
Garnet–phlogopite–pyrrhotite–graphite–quartz–anorthiterock was encountered in the borehole NV-1 at a depth ofc. 1 400 m (Fig. 2d). The Český Krumlov Varied Groupwas represented here by calcite marbles, calc-silicate
rocks, graphite–biotite gneisses and amphibolites. Geology and petrology of the borehole section was describedby Fediuková (1985).

3.5. Šebanov village 11 km SW of Český
Krumlov
Blocks of fine-grained garnet–quartz rock, locally withminor calcic amphibole, cummingtonite and magnetite
bands, occur in fields at local elevation 2.2 km south ofthe village of Šebanov (Fig. 2e). They are accompaniedby amphibolite with lenticular aggregates of dark brown
Ti-andradite up to 1 cm across, diopside–hedenbergite
and local scapolite, calc-silicate gneiss and a phlogopite–diopside rock.

2 cm
Fig. 4 Dark brown Ti-andradite aggregates in impure marble withhedenbergite/diopside and epidote. Sample CV1, locality 1, Pacova
hora near Chýnov.

362

The common feature of all above-described occurrences of Mn-rich metasediments is an association withamphibolites, marbles, graphite gneisses and quartzitesof the Varied Group. In the Český Krumlov Varied Group
calcitic and dolomitic marbles and calc-silicate gneisses
associated with graphite gneisses contain c. 2 to 10
vol. % scapolite with 2.4–2.6 wt. % Cl, which indicatesformation from sediments containing halite. The protolith
rocks are interpreted as an evaporate-bearing sequenceManganese-rich garnet–quartz rocks and gneisses, Moldanubian Zone

1 cm

Fig. 5 Dark brown Ti-andradite aggregates (Adr) with epidote (Ep) in banded
epidosite–calcite marble (Cal). Locality 1,
Pacova hora near Chýnov.

deposited in a lagoon-like environment (Kříbek et al.
1997). A similar Na/Ca scapolite was documented in calcsilicate gneiss from the Pacova hora quarry near Chýnov.

4. Analytical methods
Major-element whole-rock analyses were done by wet
chemical methods in the laboratory of the Czech Geological Survey, Prague. Minor and trace elements in somewhole-rock samples were determined by XRF and/or by
ICP-MS in the same laboratory.
Mineral analyses were obtained using CAMECA SX
100 WDS electron microprobe at the Joint Laboratory of
the Institute of Geological Sciences, Masaryk University
and Geological Survey, Brno. The analytical conditions
varied according to the mineral analysed, but usually
involved 15 kV of accelerating voltage, probe current of
10–20 nA, and acquisition time of 10–30 s. The standards
used were spessartine (Si, Mn), almandine (Fe), andradite(Ca), MgAl2O4 (Mg), hornblende (Ti), sanidine (Al, K),
albite (Na), fluorapatite (P) and chromite (Cr). Someanalyses were carried out with the CamScan 3400 electron microscope using energy dispersion analyser Link
ISIS at the Czech Geological Survey, Prague. Analyticalconditions were in this case 2.5 nA, 15 kV, 60 s counting
time on sample and 120 s on standard.
Data handling, recalculation and plotting were performed by the R language package GCDkit (Janoušek et
al. 2006). Mineral formulae recalculation used largely
worksheets presented on Web by A. Tindle. Especiallythe set for amphiboles, based on Mogessie et al. (1990)
procedure, was useful, as it provides for determinationof ferrous/ferric ratio. Mineral abbreviations in this paper
follow Kretz (1983).

5. Petrography and mineral chemistry

5.1. Chýnov
Banded garnet–quartz–plagioclase ± hornblende rock,sample BD60a, directly at contact with amphibolite(Fig. 3) is composed of very fine-grained garnet (Fig. 6a)exhibiting a strong compositional zoning (Fig. 7), mainlywith (mol. %) 36.6–47.5 Alm, 26.0–30.4 Sps, 8.5–21.4
Grs, 11.1–13.8 Prp, and 0.2–6.2 Adr (Tab. 2). Plagioclase
has composition of ~An46. Amphibole corresponds toferrimagnesiohornblende or ferritschermakite (Tab. 3)(Leake et al. 1997). Accessory minerals include ilmenitewith 24 mol. % pyrophanite and 5.8 mol. % hematite(Tab. 4), magnetite, pyrite and apatite.
Impure calcite marble with Ti-andradite aggregates
3–10 mm, diopsidic pyroxene and epidote (sample BD58,
Fig. 4) contains garnet with c. 70 mol. % andradite, 5.5
wt. % Al2O3 and 0.6–1.2 wt. % TiO2 (Tab. 2). Clinopyroxene
contains 0.35–0.5 Fe2+ pfu, minor Fe3+, Al and Na (Tab. 3).

5.2. Hosty–Doubrava
Loose skelet in the field. The rock contains 20–30 vol. %garnet in anhedral to subhedral grains set in granular363

Stanislav Vrána

quartz mosaic with accessory biotite. Garnet contains(mol. %) 45.9 Alm, 37.0 Sps, 14.2 Prp, and 2.9 Grs.
Associated biotite, in part altered, contains 1.9 wt. %MnO (Vrána 1977). Apatite and graphite are accessory
components.

5.3. Český Krumlov, Městský vrch graphite
depositAccording to Bouška et al. (1985), the calc-silicate rock(analysis No. 13 in Tab. 1) with 3.53 wt. % MnO also
contains c. 8 wt. % carbonates. It was not possible tolocate rock sample to this archival analysis.

5.4. Novosedly, NV-1 borehole, 1 420 m
Garnet–phlogopite–pyrrhotite–graphite–quartz–anorthiterock features nearly complete sulphurization of iron.

(a)

Phlogopite is represented by a nearly colourless variety
containing only 1.50 wt. % FeOt. Garnet forms colourless to whitish lenticular porphyroblasts up to 7 mm
long, which contrast with the dark, graphite-rich matrix.
Garnet contains (mol. %) 53.0–72.4 Sps, 26.3–43.6 Grs,
0.8–1.2 Prp and 0–2.7 Alm (Tab. 2). The rock contains
11 wt. % graphite and c. 5 wt. % pyrrhotite and plagioclase of bytownite–anorthite (An 89) composition. Hostrocks include calcitic and dolomitic marbles with layersof graphite gneiss, calc-silicate gneiss, paragneiss andamphibolite.

5.5. Šebanov
Garnet–quartz rock shows quartzitic layers with minor
garnet alternating with Grt-rich layers grading to several mm thick laminae of garnetite (Fig. 6c). Garnetcomposition corresponds to (mol. %) 38.5–41.3 Alm,

(b)

1 mm

0.2 mm

(c)

(d)

1 mm

2 mm

Fig. 6 Microstructures of Mn-rich garnet–quartz rocks (a–c): a – sample BD60a, Chýnov, BSE image; b – sample TYV108, Hosty–Doubrava;c – sample VE127b, Šebanov; d – Ti-andradite–hedenbergite–plagioclase rock associated with amphibolite, sample VE127e, Šebanov. Figs b–d
are photomicrographs using plane polarized light.

364

Manganese-rich garnet–quartz rocks and gneisses, Moldanubian Zone

Tab. 2 Composition of garnet (wt. % and apfu)
localityChýnovChýnovChýnovsample

BD60a

BD60a

BD58

Chýnov Doubrava Novosedly NovosedlyŠebanovŠebanovŠebanovŠebanov

BD129

BD129

VE127b

VE127b

VE127e

core

rim

1

2

VE127e

1

2

34

37

core

SiO2

37.55

37.67

35.62

35.99

rim

38.1

37.58

38.65

37.87

37.41

36.02

36.47

TiO2

0.39

0.00

1.22

0.65

n.d.

0.10

0.10

0.02

0.02

0.56

0.96

BD58 TYV108*

Al2O3

19.45

19.86

5.65

5.54

20.8

20.51

21.41

20.44

20.53

4.97

6.54

Fe2O3*

2.01

1.55

18.27

18.51

0.0

0.00

0.00

1.38

1.29

19.23

17.11

FeO*

19.81

17.23

6.28

6.49

20.2

0.00

0.80

18.06

19.33

7.55

7.48

MnO

13.14

11.34

0.15

0.21

16.1

31.01

23.61

14.97

14.64

0.92

0.66

MgO

3.21

11.34

0.19

0.13

3.5

0.30

0.20

3.47

3.58

0.09

0.16
30.60

CaO

5.04

9.00

31.91

31.91

1.0

10.10

15.34

4.98

3.93

30.69

Na2O

0.00

0.04

0.00

0.00

n.d.

0.00

0.00

0.05

0.01

0.00

0.00

Total

100.60

99.41

99.29

99.46

99.7

99.60

100.11101.19100.73100.04

99.98

Si

2.999

3.014

2.978

3.006

3.048

3.024

3.031

2.994

2.979

3.012

3.016

Al

iv

0.001

0.000

0.022

0.000

0.000

0.000

0.000

0.006

0.021

0.000

0.000

Al vi

1.840

1.880

0.563

0.573

1.964

1.946

1.983

1.906

1.911

0.516

0.667

Ti

0.023

0.000

0.077

0.041

0.000

0.006

0.006

0.001

0.001

0.035

0.060

Fe3+

0.121

0.093

1.149

1.164

0.000

0.000

0.000

0.082

0.077

1.210

1.065

Fe2+

1.324

1.153

0.439

0.454

1.369

0.000

0.079

1.194

1.287

0.528

0.517

Mn

0.889

0.769

0.011

0.015

1.091

2.114

1.568

1.003

0.987

0.065

0.046

Mg

0.382

0.329

0.024

0.016

0.417

0.036

0.023

0.409

0.425

0.011

0.020

Ca

0.431

0.772

2.858

2.856

0.084

0.871

1.289

0.422

0.335

2.750

2.712

Total

8.010

8.010

8.120

8.126

7.973

7.997

7.978

8.017

8.024

8.128

8.103

Almandine

41.8

36.6

0.0

0.0

45.9

0.0

2.7

38.5

41.3

0.0

0.0

Andradite

6.2

4.7

67.4

68.0

0.0

0.0

0.0

4.1

3.9

71.2

62.6

Grossular

8.5

21.4

31.3

30.7

2.9

26.3

43.6

10.0

7.4

25.8

34.9

Pyrope

13.1

11.2

0.9

0.6

14.2

1.2

0.8

13.7

14.3

0.4

0.8

Spessartine

30.4

26.1

0.4

0.6

37.1

72.4

53.0

33.6

33.1

2.6

1.8

Atoms / 12 oxygens

48

10 µm



Sps





18

20



16

Grs
Prp

10

12

14

mol. %

24

26

44

46

Alm



0.0 2.0

10 µm

Adr



48

Fig. 7 Compositional zoning of garnet in amphibole–garnet–quartz–plagioclase banded rock, sample BD60a, Chýnov.
46

Alm

24

26

44

365





Sps




Stanislav Vrána

Tab. 3 Composition of amphiboles and clinopyroxenes (wt. % and apfu)
LocalityChýnovChýnovChýnovChýnovChýnovŠebanovŠebanovŠebanovŠebanov

Sample

BD60a
Hbl 1

BD57
Hbl 46

BD57
Hbl 47

BD58
Cpx 1

BD58
Cpx 2

VE127bHbl 35

VE127bCum 32

VE127bCum 33

VE127e
Cpx 6

SiO2

45.12

45.26

43.61

48.09

51.21

47.52

53.75

54.29

45.33

TiO2

0.51

0.46

0.60

0.08

0.03

0.66

0.12

0.10

0.32

Al2O3

10.20

10.08

12.39

1.82

0.58

8.27

0.00

1.55

4.23

Cr2O3

0.00

0.00

0.03

0.00

0.00

0.00

0.00

0.00

0.00

V 2O 3

n.a.

0.09

0.12

0.00

0.00

0.06

0.00

0.02

0.00

Fe2O3*

11.63

17.20

11.90

4.15

1.96

14.23

0.00

0.00

7.32
11.42

FeO*

5.91

0.41

5.37

15.07

11.03

2.97

19.20

18.96

MnO

1.50

1.40

1.13

0.16

0.20

1.79

4.11

4.29

0.42

MgO

10.92

12.50

11.10

6.38

10.72

12.71

16.78

17.21

6.63

CaO

10.51

9.76

10.79

22.20

23.90

10.36

1.76

1.53

21.46

Na2O

1.02

1.13

1.45

0.95

0.43

0.89

0.10

0.11

1.43

K 2O

0.23

0.15

0.24

0.00

0.00

0.24

0.04

0.00

0.00

P 2O 5

n.a.

n.a.

n.a.

0.02

0.03

0.00

ZnO

n.a.

0.01

0.04

0.07

0.00

0.09

BaO

n.a.

0.06

0.00

0.00

0.00

0.07

F

n.a.

0.14

0.14

0.11

0.03

0.02

H 2O *

2.05

2.01

1.99

2.06

2.01

2.06

–0.06

–0.06

99.60

100.60100.84

O=F
Total

98.90

100.06

–0.05

–0.01

–0.01

101.91

97.92

100.2098.56

Atoms / 23 oxygens (amphiboles); 6 oxygens (clinopyroxenes)
Si

6.607

6.495

6.323

1.929

1.965

6.754

8.015

7.882

1.834

AlIV

1.393

1.505

1.677

0.071

0.026

1.246

0

0.118

0.166

Al

0.368

0.199

0.440

0.015

0.000

0.139

0

0.147

0.036

VI

Cr

0.003

Ti

0.056

0.050

0.065

0.002

0.001

0.071

0.013

0.011

0.010

Fe3+

1.282

1.857

1.298

0.125

0.056

1.522

0.000

0.000

0.223

2+

Fe

0.724

0.049

0.651

0.505

0.354

0.353

2.394

2.302

0.386

Mn

0.186

0.170

0.139

0.005

0.007

0.215

0.519

0.528

0.014

Mg

2.384

0.381

0.613

2.693

3.730

3.725

0.400

Zn

2.674

2.399

0.001

0.004

0.007

0.010

Ca

1.649

1.501

1.676

0.954

0.983

1.578

0.281

0.238

0.930

Na

2.290

0.314

0.408

0.074

0.032

0.245

0.029

0.031

0.112

K

0.043

0.027

0.044

0.044

0.008

0.000
0.009

0.064

0.064

0.049

0.014

OH*

F
2.000

1.936

1.936

1.951

1.986

1.991

Total

16.982

16.842

17.12716.86716.98916.992

4.061

4.037

4.111

* ferrous/ferric ratio, H2O and OH contents calculated from stoichiometry
Sample BD57 is an aliquot (paratype) to sample BD60a

33.2–33.6 Sps, 13.7–14.3 Prp, 7.4–10.0 Grs, and 3.9–4.1
Adr (Tab. 2). Accessory calcic hornblende is ferrimagnesiohornblende with c. 0.2 Mn pfu. Minor manganoan
cummingtonite contains 2.3 Fe2+ pfu and 0.5 Mn pfu (Tab.
3). Accessory ilmenite carries increased Mn corresponding to 32 to 34 mol. % pyrophanite, and minor hematite(Tab. 4). Accessory magnetite has nearly ideal simple
366

composition. Dark brown andradite-rich aggregates inamphibolite (Fig. 6d) carry garnet with c. 70 mol. %andradite, 0.6–1.0 wt. % TiO2 and 5.0–6.5 wt. % Al2O3(Tab. 2). Associated clinopyroxene contains 0.39 Fe2+ pfu,
0.40 Mg pfu, 0.22 Fe3+ pfu, slightly increased Al and Na(Tab. 3). There is also abundant amphibole, which was
not analyzed, and Ca-rich scapolite.

Manganese-rich garnet–quartz rocks and gneisses, Moldanubian Zone

Tab. 4 Composition of ilmenite and magnetite (wt. % and apfu)
Mineralilmenitemagnetitemagnetiteilmenite

Sample

BD60a

BD60a

CV1

VE127b

SiO2

0.19

0.10

0.00

0.01

TiO2

48.65

0.05

0.00

51.37

Al2O3

1.18

0.00

0.04

0.00

Cr2O3

n.a.

n.a.

n.a.

0.00

V 2O 3

n.a.

n.a.

n.a.

0.03

Fe2O3*

6.16

66.63

67.11

1.82

FeO*

32.48

30.23

30.18

30.17

FeOt

38.02

90.19

90.57

31.80

MnO

11.35

0.08

0.04

15.66

MgO

0.00

0.00

0.00

0.10

CaO

0.00

0.00

0.00

0.00

Na2O

0.00

0.00

0.00

0.00

Nb2O5

n.a.

n.a.

n.a.

0.02

UO2

n.a.

n.a.

n.a.

0.02

Total

99.4

97.09

97.37

99.07

Atoms
Si

0.005

0.004

0.000

0.000

Ti

0.920

0.001

0.000

0.982

Al

0.035

0.000

0.000

0.000

Fe

3+

0.116

1.989

2.000

0.035

Fe2+

0.683

1.003

0.999

0.642

Mn

0.242

0.003

0.001

0.337

Mg

0.000

0.000

0.000

0.004

Mol. %
Ilm

67.9

64.1
33.7

Pyr

24.0

Gei

0.0

0.4

Hem

5.8

1.7

Crn

1.7

0.0

6. Whole-rock compositional relations

6.1. Gneissic rocks
Major-element whole-rock analyses of manganiferous
rocks are presented in Tab. 5, analyses of trace elements
in Tab. 6. Normative Ab, Or and An values (Catanorm)
calculated from whole-rock analyses of manganiferous
rocks are shown in comparison with similar values forparagneisses of the Varied Group (n = 6, René 2006) in
Fig 8. The Mn-rich rocks are relatively enriched in Ca
and yield lower normative orthoclase.
Multielement diagram (Fig. 9) presents comparisonof whole-rock composition of manganiferous rocks with
Moldanubian paragneisses of the Varied Group. Averageof six paragneiss analyses published by René (2006) isused in the normalization. Abundances of SiO 2, TiO2,

Al2O3, FeOt , MgO in mangani­ferous rocks are similar to theilmenitemagnetite
paragneiss values, CaO and
VE127b
VE127b
P2O5 are moderately to strongly
0.02
0.02
elevated, but MnO enrichment
50.81
0.04
of twenty to hundred times is by
0.01
0.06
one order of magnitude higher
0.00
0.01
than that of CaO and P2O5. So0.00
0.17
dium and potassium are slightly
or very strongly depleted, indi2.42
66.74
cating a decrease or absence of
30.33
30.10
feldspars in the detrital mate32.51
90.15
rial. The tendency to Na2O and
14.90
0.08
K2O decrease is most prominent
0.10
0.01
in Mn-garnet quartzite VE127
0.00
0.00
from Šebanov (Fig. 9).
0.00
0.00
The REE distribution pat0.08
0.00
terns for three samples of
0.00
0.00
manganiferous rocks (Chýnov,Šebanov and Novosedly) (Fig.
98.68
97.05
10) are compared with the REE
abundances in the Moldanu0.001
0.001
bian paragneisses of the Varied
0.974
0.001
Group (René 2006). The rela0.000
0.003
tions strongly indicate supply
0.047
1.993
of detrital material composi0.648
0.999
tionally similar for both the
0.323
0.003
manganiferous rocks and parag0.004
0.001
neisses of the Varied Group.
The garnet–quartz rock fromŠebanov is somewhat different
64.8
in having a higher La, lower Sm
32.3
and a steeper linear trend of the
0.4light REE, reflected by ratios
2.3
LaN/YbN = 17.90 and LaN/SmN
0.0= 11.34 (Tab. 6).
A remarkable (c. 15 to 100fold) decrease in the FeOt /MnO ratio, compared to ordinary biotite gneisses of this unit, is the prime feature of
Mn-rich rocks (Tab. 5). The high Mn and increased Ca
and P contents correspond to a significant role of chemogenic components, whereas the signature of detrital material similar to that supplied for sediments metamorphosed
to paragneisses implies a mixed detrital–chemogenic
origin of the Mn-rich rocks. Composition of Mn-garnet–
quartz rocks indicates that the detrital material was nearto quartz-rich sand with a clay component.

6.2. Amphibolites in the Chýnov area
According to Janoušek et al. (2008), several amphibolite
samples from the Chýnov quarry have a relatively highratio Fe2O3/FeO = 0.7–1.0 (wt. % values). This is causedby the occurrence of accessory magnetite, the presence367

Stanislav Vrána

amphibolite layers could have formed via metamorphismof basaltic tuff – i.e., material which would be even more
susceptible to early post-deposition oxidation. The possibility of submarine alteration of basalts under shelf conditions is independently indicated by the strongly oxidic
calc-silicate assemblage andradite + epidote + diopside +magnetite, interbanded with calcite marble in close proximity to some amphibolites (Litochleb et al. 2004). Thinlayers of white quartzite and quartzitic gneiss in calcite
and dolomite marbles carrying amphibolites (Orlov 1931)
also support the idea of deposition in a shallow sea.

An
5

4
3
1

7. DiscussionAbOr

Mn-rich rocks with locality number
paragneisses of the Varied Group (René 2006)

100

Fig. 8 Comparison of normative feldspar components in Mn-rich rocks
with values for paragneisses of the Varied Group, calculated as Catanorm (Janoušek et al. 2006). Paragneiss data are from René (2006).

10

x

1

x
x

x

x

x
x

0.1

x

x

BD60a

x 13
BD129

0.01

Sample / Moldanubian paragneiss

x

VE127
SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5

Fig. 9 Multielement plots for the major and minor elements of the Mnrich rocks normalized by average compositions of the Varied Groupparagneisses (n = 6, René 2006).

of c. 10 vol. % of unmixed hematite lamellae in accessory ilmenite and probably by elevated Fe2O3/FeO ratio
in amphiboles. These features may indicate a significant
oxidation of protolithic basalts extruded on land surface
or in shallow-water, shelf environment (Janoušek et al.
2008). A probable minor increase in the oxidation state
during metamorphism is indicated by partial replacement of pyrrhotite by magnetite. Alternatively, some368

7.1. The absence of Mn-rich rocks in
Moldanubian domains of D2 and D3
refoliation
The Mn-rich rocks tend to occur in relict domains oflimited extent with preserved D1 structures and NW–SE
trending foliation. It is suggested that in regionally prevalent areas dominated by superimposed refoliation (D2, D3)(Vrána 1992; Vrána and Šrámek 1999; Vrána and Bártek
2005) the thin layers of Mn-rich rocks could have been
reduced to boudins dispersed in refoliated paragneiss.
Accidental sampling of such a boudin seems unlikely,
even though the presence of minor cummingtonite in afine-grained garnet–quartz rock could serve as indication of a former Mn-rich rock presence. Information on
crystallinity of graphite in several deposits exploited in
the past and on other graphite occurrences (Tichý 1975)
confirms the correlation of relatively coarse graphite(and graphite deposits) with relatively rare relict domains
with D1 structures still preserved. They include former
productive graphite mines near Mokrá (several km westof the area shown on Fig. 2e), Chvalovice further north(Tichý 1975), Městský vrch deposit in Český Krumlov(Fig. 2d), Hosty deposit (Fig. 2b–c), and occurrences ofgraphite quartzite and gneisses west of Černovice in theChýnov area (Suk et al. 1977). On the other hand, theBližná (Černá v Pošumaví) graphite deposit exploitedby the Václav mine till 2000 features N–S to NNE–SSW
trending structures compatible with the D2 refolding/refoliation. At this locality, a high proportion of graphite
falls in the technical “amorphous” category (Tichý 1975),
reflecting strong mylonitization of the former crystalline
graphite due to the superimposed refolding/refoliation.

7.2 Geology of marine Mn-rich sediments
The Mn-quartzites and related Mn-rich metasediments
worldwide are often associated with sea-floor basaltic
magmatism and attendant exhalative processes. In mostManganese-rich garnet–quartz rocks and gneisses, Moldanubian Zone

Tab. 5 Major- and minor-element analyses of Mn-rich garnet–quartz rocks and gneisses (wt. %)LocalityChýnov

Sample No.
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Li2O
Na2O
K 2O
P 2O 5CO2CFS
H 2O +
H 2O –(O = F)(O = S)
Total
FeOt/MnO

BD 60a
58.86
0.60
15.42
0.64
8.25
4.19
2.97
3.98
0.003
2.21
0.20
0.23
0.42
<0.01
0.03
0.47
0.94
0.03
–0.01
–0.12
99.31
2.11

Český
Krumlov
13
49.32
0.72
14.75
3.42
2.13
3.53
4.97
13.38
n.d.
0.91
1.13
0.48
3.85
n.d.
n.d.
n.d.
1.67
0.12

100.38
1.48

NovosedlyŠebanov

BD 129
47.63
0.70
13.92
<0.01
6.69
1.38
3.58
6.48
0.02
1.24
2.14
0.47
0.05
11.01
0.24
3.00
1.13
0.12
–0.10
–0.75
99.32
4.85

VE 127c
57.59
0.52
11.71
1.76
9.88
9.98
1.93
3.74
<0.001
0.01
0.12
0.16
0.08
0.02
0.01
0.02
1.74
0.11

Varied Groupparagneisses(n = 6)
59.23
0.80
16.76
8,31
0.10
3.99
1.74
2.71
3.37
0.12

LOI 2.69

99.38
1.15

99.82
74

BD60a
BD129

100

VE127

10

Sample / REE chondrite

cases there is a close correlation
with coeval basaltic (tholeiitic)
volcanism on sea floor, ventingof oceanic crust, significant enrichments in Mn transported by
upwelling sea water, typicallyin shallow sea and in proximity to transgression zones ofmarine sediments on continental crust. The sources of Mnrich exhalites are interpreted asoriginating in basic volcanics
undergoing pseudohydrothermal alterations, in hydrothermal solutions associated withvolcanism and in part in halmyrolysis of sediments during
diagenesis (Wonder et al. 1988).
The fractionation of Mn from Fe
is explained by early precipitation of Fe, relative instabilityof Mn sulphide and differencesin behaviour of Mn and Fe un-

1000

BD 60a – hornblende–plagioclase–garnet–quartz gneiss
13 – calc-silicate gneiss with high-Ca pyroxene and calcite
BD 129 – phlogopite–pyrrhotite–graphite–quartz–anorthite gneiss with garnet
VE 127c – garnet–quartz rock
Paragneisses (n = 6) – average composition of paragneisses of the Varied Group (René 2006)** total iron as Fe2O3

der changing redox conditions(Wonder et al. 1988).
Banded Mn-rich quartzites
consisting of mm to cm thickbands rich in garnet alternatingwith quartzitic (or quartz–feldspathic) laminae are interpreted
as having formed by dominant
exhalite material (spessartinerich bands) and continent-derived detritus, mostly quartz andAl-rich minerals.
Manganese formations areoften accompanied by oxidic
iron formations in condensed
Proterozoic sequences depositedon continental shelves (Bühnand Stanistreet 1997). It wassuggested that increased salinity in isolated oceanic basins
resulted in increased solubilityof Mn and Fe. Similar relations
were observed in Phanerozoic,
whereas in the present-day setting the oceanic sources andprecipitation areas of long-termpreservation are widely separated in space (Bühn and Sta­nistreet 1997).

La

Ce

Pr

Nd

Pm Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb Lu

Fig. 10 Distribution patterns of the rare earth elements in manganese-rich rocks. Chondrite values according to Boynton (1984). Grey field corresponds to variation in six paragneiss samples of the VariedGroup (René 2006).

369

Stanislav Vrána

Tab. 6 Comparison of selected trace-element concentrations in Mn-richrocks and average Moldanubian paragneisses (ppm)
Sample

BD60a

VE127

BD129

paragenisses (n = 6)*

Rb

6

3

166

148

Sr

300

2

1000

184

Zr

91

143

93

204
36

Y

35

40

43

Nb

8

6

9

Sn

<2

<2

<2

Mo

3

<1

14

Cr

72

119

98

131

Ni

42

28

48

44

Cu

21

12

78

Zn

47

47

132

Pb

4

<2

12

As

<1

<1

2

U

<2

<2

3

5

La

57.3

97.7

46.7

51.3

Ce

120.5

126.2

91.1

102.9

Pr

11.32

11.12

10.28

11.7
45.9

Nd

46.3

42.1

43.8

Sm

7.84

5.42

7.95

9.0

Eu

1.76

1.21

2.09

1.7

Gd

7.36

7.36

8.52

8.2

Tb

1.11

1.08

1.38

1.3

Dy

6.14

6.25

8.22

7.6

Ho

1.21

1.25

1.71

1.5

Er

3.54

3.87

4.57

4.4

Tm

0.47

0.55

0.65

0.7

Yb

3.23

3.68

4.16

4.0

Lu

0.47

0.55

0.58

0.6

LaN/YbN

11.96

17.90

7.57

8.65

LaN/SmN

4.60

11.34

3.70

3.59

Eu/Eu*

0.89

0.59

0.78

0.61

* average for paragneisses of the Varied Group (René 2006)

Metamorphosed Neoproterozoic Mn-rich rocks of
the Damara Belt in Namibia are associated with ironformation and supermature metaquartzarenites resting
unconformably on the pre-Damaran basement (Bühn etal. 1992). The facial evolution indicates transgression
on subsiding shelf, starting with quartzarenites followedby chemical sediments carrying ore-bearing Mn-richhorizon, overlain by carbonates and pelitic schists. Extensive amphibolite belt (former sea-floor basalts) occurs
at some distance. Garnet–quartz rocks rich in Mn, including banded types, are associated with several Mn-richlithologies including ore-grade types (Bühn et al. 1992).
Palaeozoic Mn-rich rocks associated with sulphide
deposits and iron formations in the Appalachians ofGeorgia, USA, occur in a thrust sheet comprised of metabasalts, volcanogenic alteration products and a quartzite
370

member (Wonder et al. 1988). Banded spessartine-richgarnet–quartz rocks (coticules) are a dominant rocktype of the local Mn-formation. Garnet-bearing ironformations were interpreted as a hybrid between Fe and
Mn–Fe formations. According to Wonder et al. (1988)
the garnet– quartz rocks (coticules) contain between 20
and 50 vol. % of hydrothermal material (exhalite), therest being pelagic–terrestrial detrital material. Thomson(2001) tried to assess the proportions of hydrothermal,hydrogenous and pelagic material in Mn-garnet–quartz
metasediments of two formations in New England usinggeochemical criteria. The results indicate that deciphering
the likely protoliths for individual coticule occurrences
is difficult. The absence of negative Ce anomaly is considered as excluding the role of hydrothermal processes(related to sea-floor vents). Also it became clear that
coticule geochemistry does not appear to be unique toa particular formation and may be variable within a formation, or even in individual horizon (Thomson 2001).
Differences in coticule compositions do not necessarily
preclude stratigraphic correlation.
Discussion of relations of greenschist- to amphibolite‑facies Mn-rich garnet–quartz rocks to gondites, whichwere metamorphosed mainly under granulite-facies conditions, as well as to ore-grade Mn-rich rocks, is beyond
the scope of the present study.

7.3. Potential informations on early stages ofmetamorphic crystallization in Mn-richgarnetiferous rocks
In manganiferous rocks, the crystallization of spessartinerich garnet starts at significantly lower temperatures thanwould correspond to an appearance of almandine-dominated garnet as index mineral in the Barrovian sequenceof metamorphic zones. In the Venn–Stavelot Massif, Upper Salmian (Lower Ordovician) of the Belgian Ardennes,
spessartine with hydrogarnet component appeared earlyin carpholite-bearing metapelites at conditions estimatedto ~300 °C and 0.1–0.2 GPa (Theye et al. 1996). In several areas of Lower Palaeozoic in the eastern USA, thegarnet–quartz rocks (coticules) occur in the chlorite zone
(Thomson 2001).
In garnet–quartz rocks of the Moldanubian Zone ofsouthern Bohemia, the fine-grained garnet could possibly
date from early stages of regional metamorphism. However, low-Ca samples from Hosty–Doubrava and Šebanov
localities contain garnet with minimal compositional variation (Prp ~ 13–14 mol. %), in line with the presence ofamphibolite-facies mineral assemblages in the surrounding rocks. Compositionally zoned garnet in relatively Carich sample of finely banded amphibole–quartz–plagioclase–garnet rock from Chýnov (Figs 6a, 7), with 11–13
mol. % Prp, can also be interpreted as compatible with

Manganese-rich garnet–quartz rocks and gneisses, Moldanubian Zone

amphibolite-facies conditions. The mechanism leading toa very strong zoning in the grossular component, (core 10
mol. %, rim 20 mol. %) (Fig. 7), is not fully understoodat present. It can possibly reflect a significant pressure
increase in the course of garnet crystallization. This situation would indicate either re-equilibration of early lowP–T mineral assemblage, or possibly a quick onset of theamphibolite-facies conditions. The assemblage kyanite +zoisite in local aluminous pods in calcite marble in theChýnov quarry, retrogressed to plagioclase + corundum
during the decompression stage (Vrána 1991), indicatesincreased pressure conditions (min. 0.7–1.0 GPa ?).

7.4. Role of redox conditions in Mn enrichment of the Moldanubian metasediments
The studied manganiferous rocks include both magnetite- (garnet–quartz rock from Šebanov, banded amphibole–plagioclase–quartz–garnet rock from Chýnov)and graphite-bearing types. Garnet–phlogopite–pyrrhotite–graphite–quartz–anorthite rock from the Novosedly
borehole contains 11 wt. % carbon (graphite) and c. 6
vol. % pyrrhotite (3 wt. % sulphur). The rock bears only
1.38 wt. % MnO but the colourless Sps–Grs garnet freeof iron contains up to 72 mol. % of spessartine component. This is not the single example of Mn-rich garnet ingraphite-rich rock in the Bohemian Massif. Novák (1988)
described spessartine-rich garnet from graphite in theamphibolite-facies deposit Velké Tresné near Olešnice
(Moravian Zone).
Clearly, a larger sample set would be necessary toevaluate the role of redox conditions in the studied area.
Data from the Varied Group indicate that the Mn enrichment took place both under oxidizing and reducing
conditions: the Mn-garnet–quartz rocks are oxidic but
graphite-rich gneiss BD129 crystallized under reducing
conditions.

7.5. Manganese-rich rocks in the Moldanubian Zone of western Moravia
Two occurrences of Mn-rich rocks in the MoldanubianZone of western Moravia, although different in mineralogy from the studied localities in southern Bohemia,
compare well by both their association with the VariedGroup and only very limited extent (also forming layers
less than 1 m in thickness).
(1) Metamanganolite rocks associated with minor
quartzite occur near Kojetice, about 4 km west of theTřebíč Pluton, western Moravia. According to Novák
and Škoda (2007) several types of Mn-rich rocks arepresent at the locality: a) braunite–quartz rock with minor
hyalophane, spessartine (+ accessory magnetite–rutile

intergrowths, pyrophanite, ilmenite, chernovite-(Y) andarsenogoyazite), b) Mn 3+-rich andalusite with rims of
kanonaite in quartzitic rock with braunite, K-feldspar,
albite, hyalophane, spessartine and paragonite (+ accessory minerals), c) garnet-rich rock dominated by Fe-richspessartine with quartz and accessory minerals, and d)
rhodonite-dominated rock forming layers in quartzite
only 2 mm thick. This complex group of mineral assemblages developed in a thin quartzite band, which is estimated to less than 0.5 m in width. The metamanganolite
is intercalated in weakly migmatized sillimanite–biotite
paragneiss, carrying layers of graphite gneiss with V-richmuscovite, calc-silicate rock, calcite/dolomite marble andorthogneiss.
(2) Minor skarn-like rock composed of manganoan
hedenbergite, minor spessartine–grossular garnet andbustamite with accessory sphalerite, galena and chalcopyrite occurs in the Varied Group at Meziříčko nearŽeletava, 25 km WSW of Třebíč, western Moravia (Houzar and Šrein 1995; Houzar et al. 2005). The rocks form
boudins in calcite marble, 5 cm thick, incorporated withother lithologies such as quartzite, graphite gneiss, amphibolite and pyroxene gneiss in the complex of sillimanite–biotite paragneiss and cordierite–biotite migmatite.
It is interesting to note that regional pyroxene–garnet
skarns (rich in Fe and Ca) in western Moravia and Lower
Austria show a weak tendency to Mn-enrichment, as reported by Houzar and Šrein (1995). This type of regional
garnet–pyroxene skarns is different from the Meziříčko
Mn-rich skarn in that they occur as much larger bodies(up to 300 m thick), locally mineralized with magnetite. Skarns from near Županovice contain on average
2.0 wt. % MnO in high-Ca pyroxenes; the pyroxenesof the Kottaun skarn in Lower Austria show 2.2 wt. %MnO (Houzar and Šrein (1995). Such a tendency to Mn
enrichment has not been observed in regional skarns in
the northeastern part of the Moldanubian Zone (Němec
1991; Pertoldová et al. 2009) or in southern Bohemia.
According to Pertoldová et al. (2009), garnet–pyroxene
skarns originated by regional metamorphism of protoliths
formed by mixing of detrital material and syn-sedimentary exhalites.

7.6. Comparison with Neoproterozoic Mnrich horizons in the Chvaletice area
The stratabound Chvaletice Mn–Fe deposit occurs in theeastern part of the Teplá–Barrandian Unit, eastern Bohemia, hosting the largest concentration of Mn in the Bohemian Massif. According to Cháb et al. (1982) and Žák(1982), the deposit is formed by iron meta-manganolites,
mostly of carbonate to silicate–carbonate varieties, richin pyrite, and by graphitic phyllites. The deposition took
place in a shallow-water reducing environment with mini371

Stanislav Vrána

mal supply of detrital material from crustal sources, withpossible participation of biochemogenic processes, underinfluence of simultaneous volcanism. The differences inage, redox conditions, and associated rock types place
the Chvaletice deposit in contrast to the studied Mn-richrocks in the Moldanubian Zone of southern Bohemia.

7.7. Significance of Mn-garnet–quartz rocks(coticules) in stratigraphic correlations
Since the Roman times, spessartine-rich whetstones were
exported from the Belgian Ardennes as highly valued
sharpening tools. The term coticule, derived from Latin
word for whetstone, became ingrained in geological andpetrographical terminology, especially in the easternstates of USA, Great Britain and Ireland. Schiller andTaylor (1965) explored the use of the term coticule in
the geological literature of USA since 19th century tillpresent. As follows from data in Clifford (1960), Schiller and Taylor (1965), and Thomson (2001) it is used
for fine-grained Mn-garnet–quartz rocks, which closelycompare with some samples described in this study.
While Theye et al. (1996) in a study of manganiferous
rocks in Ardennes did not use the term coticule, Lamens
et al. (1986) reported in detail on occurrences of coticules(spessartine metapelites) in the Lower Ordovician in Belgium. Baijot et al. (2011) redefined the term coticule on ahistorical basis as a rock corresponding to material used
for whetstones, composed of minute spessartine crystals(5–20 μm) set in phyllosilicate matrix. Spessartine–quartz
rocks should be designated pseudocoticule according toBaijot et al. (2011). Clearly, a terminology bias occurred
during decades of study of spessartine–quartz rocks
and only future development will show which tendency
would prevail in terminology.
In any case, there is growing evidence that Mn-richgarnet–quartz rocks are important lithostratigraphy markers. Besides the type locality in the Lower Ordovician ofArdennes, Mn-garnet–quartz rocks occur in the Ordovician of southern Ireland, possibly the Isle of Man (Kennan and Morris 1999), and in numerous Lower Palaeozoic Appalachian terrains in Nova Scotia, New England(Thomson 2001), Georgia (Wonder et al. 1988), and New
Hampshire (Clifford 1960), at levels corresponding theUpper Cambrian, Ordovician and Lower Silurian. In New
England nearly continuous coticule horizons extend for
distances of up to 350 km (Thomson 2001). This indicates
their role as important lithostratigraphy markers in unitsof moderate to regional dimensions.
In the context of increasing abundance of geochronological information supporting the Cambro–Ordovician
age of the Varied Group in the Czech part of the Moldanubian Zone (Fiala et al. 1995; Drábek and Stein 2003;
Janoušek et al. 2008; Košler et al. 2011), the occurrences372

of Mn-rich garnet–quartz rocks (coticules) appear as yet
another indication of its Lower Palaeozoic age, particularly with reference to coticule localities in NW Europe
and the eastern United States.

8. Conclusions
Manganese-rich rocks represent a notable but minor
component of the Varied Group of the Moldanubian Zone
in southern and south–central Bohemia. Garnet–quartz
rocks and gneisses carrying spessartine-rich garnet are
the main Mn-rich rock types. Accessory ilmenite andminor amphiboles participated in manganese partitioning.
Garnet–quartz rocks are oxidic, as indicated by thepresence of accessory magnetite, but the gneissic rock with
Mn-rich garnet (sample BD129 from Novosedly) carries
abundant graphite. Layers of Mn-rich rocks are closely
associated with amphibolites, marbles and graphite rocks.
Highly oxidic assemblages rich in andradite (Ti-bearing),
epidote and accessory magnetite are important in marbles
and amphibolites, particularly at Chýnov. Carbonate-facies
with rhodochrosite or rhodonite is apparently absent.
The reasons why these rocks remained unnoticed
for more than a century of geological research are: a)minimal thickness of the Mn-rich layer (less than 1 m),b) poor exposure, c) limited areal extent of relict domains
with early structures (D1 generation) still preserved.
Widespread effects of penetrative shearing deformations (D 2, D 3) apparently reduced the Mn-rich layer
to rare boudins dispersed in the transposed paragneissmatrix. Such boudins would be closely similar to garnetbearing quartzitic calc-silicate gneisses with ordinary (i.e.
low) Mn contents.
The rather unique association of several unusual rocktypes in the Český Krumlov and Chýnov areas indicatesa close correlation of these two occurrences of the VariedGroup.
Acknowledgements. I wish to thank reviewers Ma­rian
Janák and Bohdan Kříbek for suggestions towards
improving the paper and the Editor-in-chief Vojtěch
Janoušek for corrections and comments that helped
greatly to improve the manuscript. Radek Škoda and Petr
Gadas, Faculty of Science, Masaryk University, Brno,
operated the electron microprobe in the course of mineral
analysis. This publication was in part supported by theGAČR project P210/11/2358.

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